In a typical drilling operation, a drill bit is rotated while being advanced into a rock formation. There are several types of drill bits, including roller cone bits, hammer bits and drag bits. Drag bits typically include a body with a plurality of arms, or blades, extending from the body. The bit can be made of steel alloy, a tungsten matrix or other material. Steel bodied bits most often have hard metal applied typically to the top, front and back surfaces of the blades to improve the life of the body. Hard metal resists erosion and corrosion of the steel during the drilling operation. Each blade supports a plurality of cutting elements that contact, shear and/or crush the rock formation in the borehole as the bit rotates to advance the borehole. There are many kinds of bits and cutters with different features and cutter configurations.
FIG. 1 is a schematic representation of a drilling operation 2. In conventional drilling operations a drill bit 10 is mounted on the end of a drill string 6 comprising drill pipe and drill collars. The drill string may be several miles long and the bit is rotated in the bore either by a motor proximate to the bit or by rotating the drill string, or both simultaneously. A pump 8 circulates drilling fluid through the drill pipe and out of the drill bit flushing rock cuttings from the bit and transporting them back up the wellbore. The drill string comprises sections of pipe that are threaded together at their ends to create a pipe of sufficient length to reach the bottom of the wellbore.
Cutters mounted on the head of the bit can be made from any durable material but are conventionally formed from a tungsten carbide backing piece, or substrate, with a front facing table comprised of a diamond material. The tungsten carbide substrates are formed of cemented tungsten carbide comprised of tungsten carbide particles dispersed in a cobalt binder matrix. The diamond table, which engages the rock formation, typically comprises polycrystalline diamond (“PCD”) directly bonded to the tungsten carbide substrate, but could be any hard material. The PCD table provides improved wear resistance, as compared to the softer, tougher tungsten carbide substrate that supports the diamond during drilling.
Cutters are received in recesses, or pockets, along the leading edges of the blades. The cutters positioned in the pockets are secured to the drill bit body typically by brazing. The bit and cutters are subjected to high contact stresses and high temperatures in the downhole environment that can result in severe wear to them both. The cutters are subject to fracture, spalling, chipping and erosion. The body is subject to erosion and corrosion, which increases the likelihood of separation of the cutter from the pocket as the material around the cutter is lost.
While drilling rock, the PCD cutter is subject to large forces. These forces are transferred to the bit body through the pocket formed in the bit body. Where the cutter is not fully supported at the rear face, the forces generated during drilling can be sufficient to pull the cutter out of the cutter pocket.
Pockets are configured to retain the cutters during operation. The pockets in the blades orient each cutter independently of neighboring cutters. Different portions of the drill bit have configurations and orientations that optimize the function of the cutters, allowing the cutters to fail the rock optimally for a particular application. Cutters in the center of the bit may be positioned with higher back rake; that is they are angled backward in relation to the rock. Because of this, the pockets completely envelop the back portion of the cutter. In another part of the bit the cutters may be positioned with less back rake and thus the back of the cutter is less recessed, or buried, in the bit body. Thus, the pockets are shallower.
PDC cutters can be configured in the bit body in pockets that have raised features at the rear of the cutter pocket to more fully support the rear face of the cutter (FIG. 4A). This is particularly common for cutters that have less back rake, as these cutters are less recessed in the bit body. The raised backing features extend above the generally smooth surface of the bit body, or blade top, and act to support and better retain the cutter. As an integral part of the body comprised of the body material, or often a combination of materials for a steel bodied bit with its steel body and erosion resistant hard metal, these extensions are limited in strength and durability. As a result, the backing can wear or erode away risking loss of the supported cutter. Moreover, worn backing portions can result in more time consuming and expensive rebuilds of the bit. In a steel bodied bit, the shape of the raised supporting features make them difficult to machine and in addition, the application of hard metal or hardfacing to increase durability tends to burn away the steel, which compromises the strength of the raised supporting element.
Bits can incorporate backing elements as separate components. Backing elements to support the cutter have been disclosed in U.S. Pat. Nos. 4,714,120 and 7,216,565. In both examples a backing element supports the back face of a cutter. In each case the backing element includes a support base that extends generally perpendicular to the central axis of the cutter and into the bit body. This creates a high stress concentration in the backing element as it transfers forces from the cutter to the bit. The cutter is subject to repeated cyclic impacts absorbed by the backing element which can generate fatigue failure at the stress concentration points. These failures are accelerated by the high operating temperatures of the bit.
Even when employing hardened materials, the service life of a bit and cutters may be limited to a few hundred feet of operation before the bit loses effectiveness and needs to be refurbished or replaced. In particular, as the bit drills, the drilling fluid with the entrained rock cuttings tends to wear away the body and the raised cutter supporting features, allowing the cutters to over-engage the rock since there is less body material available to limit the depth of cut or over-engagement. Hardfacing is often applied to the blades of the bit to increase service life.
It should be appreciated that increasing the service life of the bit and increasing the footage drilled without damage provides more efficient and profitable operation of the equipment. Accordingly, there is room in the art for improvements in the structure and construction of bits and retention of cutters in the bit.
Examples of bits and retention of cutters in bits are also disclosed in U.S. Pat. Nos. 5,431,239 and 6,302,224. The disclosures of these and U.S. Pat. Nos. 4,714,120 and 7,216,565 are incorporated by reference in their entirety for all purposes.